This invention relates to palladium based dental alloys and, in particular, to alloys for use in porcelain-fused-to-metal restorations.
Porcelain-fused-to-metal restorations consist of a metallic sub-structure coated with a veneer of porcelain. Over the years various alloys have been proposed for the sub-structure of these restorations. Many of the early alloys used gold with some platinum or palladium as the main alloy ingredients. However, with the increases and fluctuations in the price of gold and platinum in recent years, other alloys have come to play major roles in this area. One series of alloys which has gained general acceptance is based on nickel, chromium and beryllium as the main ingredients. Another series of alloys, with which this invention is concerned, is based on palladium as the dominant element.
One such palladium based alloy for use in porcelain-fused-to-metal restorations is described in U.S. Pat. No. 4,261,744. This alloy includes approximately 80% palladium and lesser amounts of indium, tin, cobalt and silicon. Another palladium alloy, which was commercially available prior to this invention, includes, based on spectrographic analysis, approximately 2% gold, 79% palladium, 9% gallium, 10% copper and perhaps a trace of boron (on the order of 0.1%). Alloys similar to this commercial alloy, which include gold, palladium gallium, copper and boron are described in U.S. Pat. Nos. 3,134,671 and 4,179,288.
In examining the commercially available gold-palladium alloy described above, it was found that the alloy suffered a number of disadvantages in terms of its suitability for use in porcelain-fused-to-metal restorations. In particular, the alloy exhibited poor grain structure which give it low elongation, lower than optimum tensile strength and low toughness, as well as making it susceptible to "hot-tearing" during the investment casting process.
Suprisingly, in seeking to overcome these limitations, numerous difficulties were encountered in attempting to grain refine this alloy. In particular, it was found that when the standard grain refining techniques were applied to the alloy, and the alloy then used to make a casting for a porcelain-fused-to-metal restoration, the casting caused bubbles to form in the porcelain during the porcelain firing process. This resulted in an unusable restoration.
Moreover it was found that the commercial gold-palladium alloy had a coefficient of thermal expansion which was not compatible with the full range of porcelains available for porcelain-fused-to-metal restorations. In particular, although the alloy could be used with porcelains having a low coefficient of thermal expansion, it could not be used with porcelains having a high coefficient, particularly for long-span bridgework involving pontics.
Accordingly, it is one of the objects of this invention to overcome the limitations of the above described commercially available palladium based dental alloy. In particular, it is an object of this invention to provide a grain refined palladium based dental alloy which will not produce bubbles when porcelain is applied. It is a further object of the invention to produce a palladium based dental alloy which has a coefficient of expansion compatible with the complete range of dental porcelains commonly used in porcelain-fused-to-metal restorations.
The attainment of these and other objects of the invention is described below in connection with the description of the preferred embodiments of the invention.